The present invention relates to a polyamide acid composition for preparing a polyimide and a process for preparing a polyimide from the same.
A polyimide has an excellent heat resistance and electric insulation, and is useful as electric and electronic parts, electric insulating materials and aircraft parts which are used at a high temperature.
However, it is difficult to mold the polyimide itself due to its excellent heat resistance. Therefore, the polyimide product is, in general, prepared by molding a solution of a polyamide acid in an organic solvent (hereinafter referred to as "polyamide acid solution") which is a precursor of the polyimide, and then converting the polyamide acid to the polyimide.
As a process for converting the polyamide acid to the polyimide there are known (A) a process by evaporating a solvent in the polyamide solution by heating, and at the same time thermally converting the polyamide acid to the polyimide; and (B) a process by chemically converting the polyamide acid to the polyimide with a dehydrating agent and a catalyst; and a process of the combination of the processes (A) and (B).
In the process (B), there are known two processes, that is, (B-1) a process by molding a polyamide acid solution, and then immersing into a converting bath prepared with a dehydrating agent and a catalyst, and (B-2 ) a process by molding a homogeneous admixture of a polyamide acid solution, a dehydrating agent and a catalyst.
Those processes have, however, the following defects. As to the process (A), the heating should be carried out quite gradually in order to avoid generation of voids, and water which yields during the conversion of the polyamide acid to the polyamide causes hydrolysis of the obtained polyimide to make charactaristic properties of the polyimide reduce. As to the process (B-1), it takes a long time to convert the polyamide acid to the polyimide, and a degree of the conversion in the surface of the molding and that of the interior are not uniform. On the other hand, the process (B-2) does not have such defects as those in the processes (A) and (B-1). In the process (B-2), however, the conversion of the polyamide acid to the polyimide is drastically accelerated just after the dehydrating agent and the catalyst are added to the polyamide acid solution. As a result, the polyamide acid is gelled in a short time. Therefore, in case of employing the process (B-2), the molding operation, particularly continuous molding operation, is difficult because the pot life of the polyamide acid composition very short.
When the dehydrating agent and the catalyst are added to the polyamide acid solution, the polyamide acid converts to the corresponding polyimide by dehydration and ring closure. In the course of the conversion, the polyamide acid is increased in viscosity, gelled, and then hardened. Accordingly, molding operation must be carried out within a period during which the polyamide acid is not yet gelled. A time of period required for molding operation is varied depending on a molding apparatus and a molding manner. In the continuous molding operation, since the polyamide acid is gelled during staying in a tube and the staying in the tube is impossible to avoid, it is preferable that the pot life which is determined by a kind of the polyamide acid, a kind of solvent, a concentration of the polyamide acid solution, and kinds and amounts of the dehydrating agent and the catalyst is longer than the time of period for molding operation. If the pot life is shorter than the time of period for molding operation, the pot life can be extended by altering kinds and amounts of the dehydrating agent and the catalyst. In that case, however, the producibility is lowered because the rate of the conversion is lowered, and the properties of the obtained polyimide are inferior to the polyimide prepared by the process (B-2).
In the above-mentioned conventional processes, in order to eliminate such defects, the polyamide acid composition is maintained at a low temperature. As a result, the activities of the dehydrating agent and the catalyst are reduced. By such manner, however, the flowability of the polyamide acid solution decreases due to increase of the viscosity of the solution by cooling, and it is difficult to maintain the solution at the low temperature due to heat generation by admixing the dehydrating agent and the catalyst to the polyamide acid solution. As a result, enough pot life cannot be obtained. Therefore, the cooling is not an effective treatment, either.